Acrylic compositions and articles made from acrylic compositions are well known for their clarity, lack of color, surface gloss, excellent processability, good scratch resistance and resistance to UV degradation. They are also well known for their low-impact strength or brittleness. It has become standard practice to incorporate toughening agents or impact modifiers in acrylic formulations to improve their mechanical properties.
One type of polymeric impact modifier is a sequentially polymerized acrylic composition. As described in U.S. Pat. No. 3,661,994, these are known as “core-shell” polymers, where either the core layer or an intermediate layer is made of a rubber polymer, and the outer-stage layer, also known as the shell layer, consists of a glassy polymer that is compatible with the acrylic polymer matrix. The core-shell polymers are formed in a latex, from which they are isolated into dry particles. Rubbery core impact modifiers typically suffer from relatively low modulus and relatively poor weatherability, which is detrimental to the impact modified acrylic formulation. U.S. Pat. No. 4,521,568 and US2003/0216510, describe core-shell polymers consisting of a hard glassy core layer, and one or more rubber intermediate layers to impact modify an acrylic polymer while retaining high modulus and good weathering resistance.
U.S. Pat. No. 7,580,151 and US 2009/0018237 describe transparent thermoplastic blends in which the refractive index of the impact modifier must be very close (within 0.005 or 0.008) of the matrix. This severely limits the possible formulation combinations to achieve a transparent product.
Surprisingly, it has been found that selected combinations of matrix polymer with impact modifier particles and/or impact modifier composite particles, having a refractive index difference of greater than 0.005, and even greater than 0.008, results in transparent, impact-modified compositions. Core-shell impact modifiers are especially preferred. By removing the narrow refractive index difference limits typically placed on materials incorporated into transparent products, the invention allows for the combination of a larger universe of materials and greater design flexibility. The refractive-index mismatched compositions can provide better dispersibility as well as improvements in physical and mechanical properties.